Conventionally, a thin film transistor (hereinafter referred to as TFT) memory including a memory TFT for storing data and a drive TFT for selectively driving the memory TFT is known as a non-volatile memory, such as an electrically erasable and programmable read only memory (EEPROM).
Examples of such a TFT memory include a TFT memory which employs a technique using an insulating film having a trapping level. In such a TFT memory employing this technique using such an insulating film, the drive TFT and the memory TFT each employ, for example, a bottom gate structure, and each include a gate electrode formed on an insulating substrate such as a glass substrate, a gate insulating film formed so as to cover the gate electrode, a semiconductor layer formed of amorphous silicon (a-Si) etc. formed on the gate insulating film so as to overlap with the gate electrode, and a source electrode and a drain electrode, spaced apart from each other, coupled to the semiconductor layer.
The memory TFT described above provides a memory function utilizing hysteresis phenomena (hysteretic phenomena), which occur from charge accumulation caused by electrical trap in the vicinity of the boundary with the semiconductor layer in the gate insulating film. Therefore, the memory TFT needs to have a sufficiently high hysteresis nature. As such, a gate insulating film having a large number of trapping levels, and accordingly having a high capability of charge accumulation, is preferably used as a gate insulating film of a memory TFT.
On the contrary, a wide variation in the threshold voltage will prevent the drive TFT described above from performing correct selective driving of the memory TFT. Therefore, the drive TFT needs to have a low hysteresis nature. As such, a gate insulating film having only a small number of trapping levels, and accordingly having a low capability of charge accumulation, is preferably used as a gate insulating film of a drive TFT.
This condition requires the memory TFT and the drive TFT to be formed in separate steps, thereby requiring numerous steps to be performed in order to form a TFT memory.
In this regard, one well known TFT memory is such that, in order to simplify the steps of forming the TFT memory described above, a common gate insulating film is used for the drive TFT and the memory TFT, and a process is performed to change the film properties of either the drive TFT region or the memory TFT region in the gate insulating film so as to partially adjust the capability of charge accumulation of the gate insulating film.
For example, the TFT memory disclosed in Patent Document 1 uses a gate insulating film formed of silicon nitride (SiN) having the capability of charge accumulation (hysteresis nature), and the capability of charge accumulation in the regions other than the memory TFT region in the gate insulating film is disabled by a nitridation or oxidation process.
The TFT memory disclosed in Patent Document 2 uses a gate insulating film formed of silicon nitride (SiN) having no capability of charge accumulation (hysteresis nature), and the memory TFT region of the gate insulating film is provided with the capability of charge accumulation by implantation of silicon (Si) ion.
The TFT memory disclosed in Patent Document 3 uses a gate insulating film formed of silicon nitride (SiN) having the capability of charge accumulation (hysteresis nature), and a silicon nitride film having no capability of charge accumulation is formed between the gate electrode and the gate insulating film of the drive TFT.
In addition, the TFT memory disclosed in Patent Document 4 uses a gate insulating film, at least a surface layer of which is formed of silicon nitride (SiN) having the capability of charge accumulation, and n-type impurity is diffused into the channel region of the drive TFT. Thus, the bandgap difference between the semiconductor layer and the gate insulating film is increased, thereby cancelling the effects of charge injection between the semiconductor layer and the gate insulating film. Accordingly, the hysteresis nature of the drive TFT is lost.